You only need to walk down your supermarket's personal care aisle to understand how big of a business the selling of deodorant and antiperspirant has become in the US, totaling over $2 billion in U.S. sales last year alone. But do you know what's actually in the stuff you roll on (hopefully) every morning?

The Origin of Sweat Stink

Your armpits play an essential role in regulating your body's temperature, their roughly three million sweat glands pumping out as much as 14 liters of water a day (or as much as 4 liters of water every hour under extreme conditions). Interestingly, while most mammals possess sweat glands very few—including horses and humans—produce copious amounts of fluid to thermoregulate.

Like urine, sweat is sterile and odorless when it is first secreted, only generating its telltale odor as the liquid is fermented by environmental bacteria. In the case of armpit perspiration, there are two distinct types of glands that produce sweat. The eccrine glands cool the body, secreting only water and electrolytes. Given the relatively nutrient-poor quality of this sweat, it rarely attracts bacteria and does not strongly impact your scent. The apocrine glands, however, also transport fats and proteins to the skin's surface along with sweat, which are then digested by bacteria colonies—along with dead skin and hair cells—to produce the sweat odor as a metabolic byproduct.

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The underarms have been compared to tropical rainforests amid the the skin's typically arid topography, which makes it an ideal home for a huge variety of bacteria. These species thrive in moist, low pH environments created when you wash off the natural acid mantle produced by your underarms with alkaline soaps. Shaving your armpits aids in bacterial growth by removing the hairs that naturally wick moisture away from the skin surface. And while the average unwashed hermit or granola-pounding hippy will tell you that this natural musk is quite lovely, it's understandable that most of us opt for the fresh scent of deodorized skin.

The Difference Between Deodorants and Antiperspirants

Deodorants and antiperspirants are not one and the same. These two chemical compounds are designed for radically divergent purposes, and function very differently when applied to the skin.

The earliest known reference to deodorants comes from 9th century Persian scholar and polymath, Ziryab, as part of his push for cleanliness and personal hygiene among the Umayyad court of Islamic Iberia along with the radical notions of bathing and toothpaste (which he reportedly invented). But it wasn't until the Victorian Era that a Philadelphia inventor (whose name has been scrubbed from history) developed the first commercial deodorant, Mum, in 1888. Bristol-Meyers acquired the company in 1931 and, a decade later, revolutionized personal hygiene by developing a roll-on applicator fashioned after ballpoint pen technology to create Ban Roll-On deodorant.

Deodorants don't do much to actually stop you from sweating, they instead target the bacteria that feed on your sweat. They often contain alcohol- or chelant-based ingredients that make your underarms inhospitable to these colonies or include antibacterial chemicals like triclosan to kill off the bacteria before they have time to digest your fluids. As such, the FDA regulates deodorants as cosmetics.

Antiperspirants, on the other hand, are classified as drugs by the FDA when combined with deodorants. They first came to market at the turn of the 20th century in the form of Everdry. However, this product quickly became troublesome due to elevated levels of aluminium chloride, which causes contact dermatitis (read: itchy, irritated skin) in a portion of the population and can be fatal if large enough concentrations seep into the body and shut down the kidneys. Jules Montenier solved this issue in 1941 when he patented the first modern antiperspirant mixture, which mitigated the aluminum chloride's negative with a soluble nitrile compound.

Aluminum chloride compounds—such as aluminium zirconium tetrachlorohydrex glyand—remain among the most effective antiperspirant agents on the market today. These compounds mix with sweat to form a gel-like plug that stops up the sweat gland duct (as well as causing it to constrict). The more pores that are plugged, the less you will physically sweat. The process is temporary of course—the plug will eventually come off with sloughing skin—though the effect does vary between individuals. Other active ingredients also regularly include parabens and Butylated hydroxytoluene (BHT), which act as preservatives; masking fragrances, moisturizing emollient oils, emulsifying agents, and talcum powder to reduce friction.

"You want your underarms to be as dry as possible so that the antiperspirant's active ingredients have a chance to do their job by seeping into pores and plugging sweat ducts," David Pariser, MD, professor of dermatology at Eastern Virginia Medical School in Norfolk, told WebMD. That's why you should put on antiperspirants at night, before going to bed, rather than in the morning just after you get out of the shower."

Antiperspirants Can Stink

As with many other modern cosmetic pharmaceuticals like sunscreen and toothpaste, too much antiperspirant might do more harm than good. As mentioned above, a small portion of the American population is allergic to aluminum, and its application can result the skin becoming itchy, red, and inflamed.

What's more, long-term usage has been linked to elevated levels of aluminum in the user's system (also known as the "body burden," similar to how fish become laden with mercury over time). Too much aluminum in your system can be fatal, as it causes your kidneys to shut down, which is why the FDA has been labeling antiperspirants with warnings against their use by people with weakened kidneys for the last decade. Other potential irritants include zirconium and propylene glycol, both of which are common antiperspirant ingredients.

That's not to say that using a reasonable amount of antiperspirant every day will cause you harm. But if you're vulnerable in certain areas—aluminum sensitivity, weak kidneys—it's worth moderating your usage

Do Antiperspirants Really Cause Cancer?

Despite popular belief, one thing antiperspirants don't do is cause cancer. That's according to a number of major medical research institutes. But since this is the Internet and you're going to prattle on about how it's all a huge conspiracy anyway, let's take a look at how the antiperspirant-cancer myth came about.

Around the turn of the 21st century, a rumor linking the an increased risk of developing breast cancer to the practice of shaving and applying antiperspirants to the underarms. In an effort to put this matter to rest, the American Cancer Society cites two studies, conducted in 2002 and 2003:

There are no strong epidemiologic studies in the medical literature that link breast cancer risk and antiperspirant use, and very little scientific evidence to support this claim.

In fact, a carefully designed epidemiologic study of this issue published in 2002 compared 813 women with breast cancer and 793 women without the disease. The researchers found no link between breast cancer risk and antiperspirant use, deodorant use, or underarm shaving.

A study published in 2003 looked at responses from questionnaires sent out to women who had breast cancer. The researcher reported that women who were diagnosed with breast cancer at a younger age said they used antiperspirant and started shaving their underarms earlier and shaved more often than women who were diagnosed when they were older. But the study design did not include a control group of women without breast cancer and has been criticized by experts as not relevant to the safety of these underarm hygiene practices.

Shortly thereafter, the Susan G. Komen Cancer Foundation, the National Cancer Institute, and BreastCancer.org all independently chimed in supporting the ACS's position. Researchers at the NCI, went as far as to say that they "are not aware of any conclusive evidence linking the use of underarm antiperspirants or deodorants and the subsequent development of breast cancer." But not all researchers were convinced.

"Absence of evidence is not evidence of absence of a harmful effect" and "these chemicals are being directly applied daily, by very large numbers of people, and the long-term health effects of exposure are essentially unknown," toxicologist Philip W. Harvey told WebMD.

In 2004 and 2005, a pair of studies conducted by Dr. Philippa Darbre and published in the Journal of Applied Toxicology and the Journal of Inorganic Chemistry, respectively, exhibited a link between the application of aluminum and unchecked DNA mutations—a prerequisite for tumor growth. A subsequent study in 2007 suggested that antiperspirants contribute to the body's aluminum burden that we discussed above.

These studies were all quickly refuted by other researchers, but not very well. ACS epidemiologist Michael Thun argued in 2008 that "studies have not shown any direct link between parabens and any health problems, including breast cancer. What has been found is that there are many other compounds in the environment that also mimic naturally produced estrogen." He continued, "even if the parabens do promote estrogen-dependent tumor growth, the risk from cosmetic use is 'minuscule' compared with other known tumor promoters."

And again in 2009, yet another study linked the use of phthalates and aluminum salts with the development of breast cancer, citing the chemicals' ability to accumulate in the body and mimic (or at least amplify) the effects of estrogen. Overall, attempts to recreate Darbre's findings have returned mixed results, leading to the current set of ambiguous circumstances as to the safety of these products.

So, as is the case with e-cigarettes, the jury is still out as to their overall safety. If you are worried that your antiperspirant will incite a cancerous growth somewhere down the line, don't go au naturale, just switch to straight deodorant. Everybody in the elevator will thank you. [CBS News 1, 2 - Statista - NCI - WebMD 1 - Wiki - ACS - KOMO News]